Regulation of IP(3)R Channel Gating by Ca(2+) and Ca(2+) Binding Proteins

Purinergic agonists increase [Ca2+]i in rat conjunctival goblet cells through ryanodine receptor type 3

ATP and benzoylbenzoyl-ATP (BzATP) increase free cytosolic Ca2+ concentration ([Ca2+]i) in conjunctival goblet cells (CGCs) resulting in mucin secretion. The purpose of this study was to investigate the source of the Ca2+i mobilized by ATP and BzATP. First-passage cultured rat CGCs were incubated with Fura-2/AM, and [Ca2+]i was measured under several conditions with ATP and BzATP stimulation. The following conditions were used: 1) preincubation with the Ca2+ chelator EGTA, 2) preincubation with the SERCA inhibitor thapsigargin (10-6 M), which depletes ER Ca2+ stores, 3) preincubation with phospholipase C (PLC) or protein kinase A (PKA) inhibitor, or 4) preincubation with the voltage-gated calcium channel antagonist nifedipine (10-5 M) and the ryanodine receptor (RyR) antagonist dantrolene (10-5 M). Immunofluorescence microscopy (IF) and quantitative reverse transcription polymerase chain reaction (RT-qPCR) were used to investigate RyR presence in rat and human CGCs. ATP-stimulated peak [Ca2+]i was significantly lower after chelating Ca2+i with 2 mM EGTA in Ca2+-free buffer. The peak [Ca2+]i increase in CGCs preincubated with thapsigargin, the PKA inhibitor H89, nifedipine, and dantrolene, but not the PLC inhibitor, was reduced for ATP at 10-5 M and BzATP at 10-4 M. Incubating CGCs with dantrolene alone decreased [Ca2+]i and induced CGC cell death at a high concentration. RyR3 was detected in rat and human CGCs with IF and RT-qPCR. We conclude that ATP- and BzATP-induced Ca2+i increases originate from the ER and that RyR3 may be an essential regulator of CGC [Ca2+]i. This study contributes to the understanding of diseases arising from defective Ca2+ signaling in nonexcitable cells.NEW & NOTEWORTHY ATP and benzoylbenzoyl-ATP (BzATP) induce mucin secretion through an increase in free cytosolic calcium concentration ([Ca2+]i) in conjunctival goblet cells (CGCs). The mechanisms through which ATP and BzATP increase [Ca2+]i in CGCs are unclear. Ryanodine receptors (RyRs) are fundamental in [Ca2+]i regulation in excitable cells. Herein, we find that ATP and BzATP increase [Ca2+]i through the activation of protein kinase A, voltage-gated calcium channels, and RyRs, and that RyRs are crucial for nonexcitable CGCs' Ca2+i homeostasis.

A Model for the Coexistence of Competing Mechanisms for Ca 2 + Oscillations in T-lymphocytes

Ca 2 + is a ubiquitous signaling mechanism across different cell types. In T-cells, it is associated with cytokine production and immune function. Benson et al. have shown the coexistence of competing Ca 2 + oscillations during antigen stimulation of T-cell receptors, depending on the presence of extracellular Ca 2 + influx through the Ca 2 + release-activated Ca 2 + channel (Benson in J Biol Chem 29:105310, 2023). In this paper, we construct a mathematical model consisting of five ordinary differential equations and analyze the relationship between the competing oscillatory mechanisms.. We perform bifurcation analysis on two versions of our model, corresponding to the two oscillatory types, to find the defining characteristics of these two families.

Disrupted Endoplasmic Reticulum Ca2+ Handling: A Harβinger of β-Cell Failure

The β-cell workload increases in the setting of insulin resistance and reduced β-cell mass, which occurs in type 2 and type 1 diabetes, respectively. The prolonged elevation of insulin production and secretion during the pathogenesis of diabetes results in β-cell ER stress. The depletion of β-cell Ca2+ER during ER stress activates the unfolded protein response, leading to β-cell dysfunction. Ca2+ER is involved in many pathways that are critical to β-cell function, such as protein processing, tuning organelle and cytosolic Ca2+ handling, and modulating lipid homeostasis. Mutations that promote β-cell ER stress and deplete Ca2+ER stores are associated with or cause diabetes (e.g., mutations in ryanodine receptors and insulin). Thus, improving β-cell Ca2+ER handling and reducing ER stress under diabetogenic conditions could preserve β-cell function and delay or prevent the onset of diabetes. This review focuses on how mechanisms that control β-cell Ca2+ER are perturbed during the pathogenesis of diabetes and contribute to β-cell failure.

Pacemaking in the lymphatic system

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

Host IP3R channels are dispensable for rotavirus Ca2+ signaling but critical for intercellular Ca2+ waves that prime uninfected cells for rapid virus spread

IMPORTANCE

Many viruses exploit host Ca2+ signaling to facilitate their replication; however, little is known about how Ca2+ signals from different host and viral channels contribute to the overall dysregulation of Ca2+ signaling or promote virus replication. Using cells lacking IP3R, a host ER Ca2+ channel, we delineated intracellular Ca2+ signals within virus-infected cells and intercellular Ca2+ waves (ICWs), which increased Ca2+ signaling in neighboring, uninfected cells. In infected cells, IP3R was dispensable for rotavirus-induced Ca2+ signaling and replication, suggesting the rotavirus NSP4 viroporin supplies these signals. However, IP3R-mediated ICWs increase rotavirus replication kinetics and spread, indicating that the Ca2+ signals from the ICWs may prime nearby uninfected cells to better support virus replication upon eventual infection. This "pre-emptive priming" of uninfected cells by exploiting host intercellular pathways in the vicinity of virus-infected cells represents a novel mechanism for viral reprogramming of the host to gain a replication advantage.

An integrate-and-fire approach to Ca2+ signaling. Part II: Cumulative refractoriness

Inositol 1,4,5-trisphosphate-induced Ca2+ signaling is a second messenger system used by almost all eukaryotic cells. The agonist concentration stimulating Ca2+ signals is encoded in the frequency of a Ca2+ concentration spike sequence. When a cell is stimulated, the interspike intervals (ISIs) often show a distinct transient during which they gradually increase, a system property we refer to as cumulative refractoriness. We extend a previously published stochastic model to include the Ca2+ concentration in the intracellular Ca2+ store as a slow adaptation variable. This model can reproduce both stationary and transient statistics of experimentally observed ISI sequences. We derive approximate expressions for the mean and coefficient of variation of the stationary ISIs. We also consider the response to the onset of a constant stimulus and estimate the length of the transient and the strength of the adaptation of the ISI. We show that the adaptation sets the coefficient of variation in agreement with current ideas derived from experiments. Moreover, we explain why, despite a pronounced transient behavior, ISI correlations can be weak, as often observed in experiments. Finally, we fit our model to reproduce the transient statistics of experimentally observed ISI sequences in stimulated HEK cells. The fitted model is able to qualitatively reproduce the relationship between the stationary interval correlations and the number of transient intervals, as well as the strength of the ISI adaptation. We also find positive correlations in the experimental sequence that cannot be explained by our model.

A dual-clock-driven model of lymphatic muscle cell pacemaking to emulate knock-out of Ano1 or IP3R

Lymphatic system defects are involved in a wide range of diseases, including obesity, cardiovascular disease, and neurological disorders, such as Alzheimer's disease. Fluid return through the lymphatic vascular system is primarily provided by contractions of muscle cells in the walls of lymphatic vessels, which are in turn driven by electrochemical oscillations that cause rhythmic action potentials and associated surges in intracellular calcium ion concentration. There is an incomplete understanding of the mechanisms involved in these repeated events, restricting the development of pharmacological treatments for dysfunction. Previously, we proposed a model where autonomous oscillations in the membrane potential (M-clock) drove passive oscillations in the calcium concentration (C-clock). In this paper, to model more accurately what is known about the underlying physiology, we extend this model to the case where the M-clock and the C-clock oscillators are both active but coupled together, thus both driving the action potentials. This extension results from modifications to the model's description of the IP3 receptor, a key C-clock mechanism. The synchronised dual-driving clock behaviour enables the model to match IP3 receptor knock-out data, thus resolving an issue with previous models. We also use phase-plane analysis to explain the mechanisms of coupling of the dual clocks. The model has the potential to help determine mechanisms and find targets for pharmacological treatment of some causes of lymphoedema.

The Inositol Trisphosphate Receptor (IP 3 R) is Dispensable for Rotavirus-induced Ca 2+ Signaling and Replication but Critical for Paracrine Ca 2+ Signals that Prime Uninfected Cells for Rapid Virus Spread

Rotavirus is a leading cause of viral gastroenteritis. A hallmark of rotavirus infection is an increase in cytosolic Ca 2+ caused by the nonstructural protein 4 (NSP4). NSP4 is a viral ion channel that releases Ca 2+ from the endoplasmic reticulum (ER) and the increase in Ca 2+ signaling is critical for rotavirus replication. In addition to NSP4 itself, host inositol 1,4,5- trisphosphate receptor (IP 3 R) ER Ca 2+ channels may contribute to rotavirus-induced Ca 2+ signaling and by extension, virus replication. Thus, we set out to determine the role of IP 3 R Ca 2+ signaling during rotavirus infection using IP 3 R-knockout MA104-GCaMP6s cells (MA104- GCaMP6s-IP 3 R-KO), generated by CRISPR/Cas9 genome editing. Live Ca 2+ imaging showed that IP 3 R-KO did not reduce Ca 2+ signaling in infected cells but eliminated rotavirus-induced intercellular Ca 2+ waves (ICWs) and therefore the increased Ca 2+ signaling in surrounding, uninfected cells. Further, MA104-GCaMP6s-IP 3 R-TKO cells showed similar rotavirus susceptibility, single-cycle replication, and viral protein expression as parental MA104- GCaMP6s cells. However, MA104-GCaMP6s-IP 3 R-TKO cells exhibited significantly smaller rotavirus plaques, decreased multi-round replication kinetics, and delayed virus spread, suggesting that rotavirus-induced ICW Ca 2+ signaling stimulates virus replication and spread. Inhibition of ICWs by blocking the P2Y1 receptor also resulted in decreased rotavirus plaque size. Conversely, exogenous expression of P2Y1 in LLC-MK2-GCaMP6s cells, which natively lack P2Y1 and rotavirus ICWs, rescued the generation of rotavirus-induced ICWs and enabled plaque formation. In conclusion, this study shows that NSP4 Ca 2+ signals fully support rotavirus replication in individual cells; however, IP 3 R is critical for rotavirus-induced ICWs and virus spread by priming Ca 2+ -dependent pathways in surrounding cells.

Importance

Many viruses exploit host Ca 2+ signaling to facilitate their replication; however, little is known about how distinct types of Ca 2+ signals contribute to the overall dysregulation of Ca 2+ signaling or promote virus replication. Using cells lacking IP 3 R, a host ER Ca 2+ channel, we could differentiate between intracellular Ca 2+ signals within virus-infected cells and intercellular Ca 2+ waves (ICWs), which increase Ca 2+ signaling in neighboring, uninfected cells. In infected cells, IP 3 R was dispensable for rotavirus-induced Ca 2+ signaling and replication, suggesting the rotavirus NSP4 viroporin supplies these signals. However, IP 3 R-mediated ICWs increase rotavirus replication kinetics and spread, indicating that the Ca 2+ signals from the ICWs may prime nearby uninfected cells to better support virus replication upon eventual infection. This "pre-emptive priming" of uninfected cells by exploiting host intercellular pathways in the vicinity of virus-infected cells represents a novel mechanism for viral reprogramming of the host to gain a replication advantage.

Viewing Ca2+-binding sites in the inositol trisphosphate receptor

Ca²⁺ is a major ligand of the inositol 1,4,5-trisphosphate receptor (IP₃R) Ca²⁺-release channel. Fan et al. [1] recently solved additional cryo-electron microscopy (cryo-EM) structures of the IP₃R in different ligand-binding states, revealing new Ca²⁺ binding sites.

Calcium and Reactive Oxygen Species Signaling Interplays in Cardiac Physiology and Pathologies

Mitochondria are key players in energy production, critical activity for the smooth functioning of energy-demanding organs such as the muscles, brain, and heart. Therefore, dysregulation or alterations in mitochondrial bioenergetics primarily perturb these organs. Within the cell, mitochondria are the major site of reactive oxygen species (ROS) production through the activity of different enzymes since it is one of the organelles with the major availability of oxygen. ROS can act as signaling molecules in a number of different pathways by modulating calcium (Ca2+) signaling. Interactions among ROS and calcium signaling can be considered bidirectional, with ROS regulating cellular Ca2+ signaling, whereas Ca2+ signaling is essential for ROS production. In particular, we will discuss how alterations in the crosstalk between ROS and Ca2+ can lead to mitochondrial bioenergetics dysfunctions and the consequent damage to tissues at high energy demand, such as the heart. Changes in Ca2+ can induce mitochondrial alterations associated with reduced ATP production and increased production of ROS. These changes in Ca2+ levels and ROS generation completely paralyze cardiac contractility. Thus, ROS can hinder the excitation-contraction coupling, inducing arrhythmias, hypertrophy, apoptosis, or necrosis of cardiac cells. These interplays in the cardiovascular system are the focus of this review.

Endocannabinoid signaling in astrocytes

The study of the astrocytic contribution to brain functions has been growing in popularity in the neuroscience field. In the last years, and especially since the demonstration of the involvement of astrocytes in synaptic functions, the astrocyte field has revealed multiple functions of these cells that seemed inconceivable not long ago. In parallel, cannabinoid investigation has also identified different ways by which cannabinoids are able to interact with these cells, modify their functions, alter their communication with neurons and impact behavior. In this review, we will describe the expression of different endocannabinoid system members in astrocytes. Moreover, we will relate the latest findings regarding cannabinoid modulation of some of the most relevant astroglial functions, namely calcium (Ca²⁺ ) dynamics, gliotransmission, metabolism, and inflammation.

Conformational motions and ligand-binding underlying gating and regulation in IP3R channel

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are activated by IP3 and Ca2+ and their gating is regulated by various intracellular messengers that finely tune the channel activity. Here, using single particle cryo-EM analysis we determined 3D structures of the nanodisc-reconstituted IP3R1 channel in two ligand-bound states. These structures provide unprecedented details governing binding of IP3, Ca2+ and ATP, revealing conformational changes that couple ligand-binding to channel opening. Using a deep-learning approach and 3D variability analysis we extracted molecular motions of the key protein domains from cryo-EM density data. We find that IP3 binding relies upon intrinsic flexibility of the ARM2 domain in the tetrameric channel. Our results highlight a key role of dynamic side chains in regulating gating behavior of IP3R channels. This work represents a stepping-stone to developing mechanistic understanding of conformational pathways underlying ligand-binding, activation and regulation of the channel.

Dysregulated Ca2+ Homeostasis as a Central Theme in Neurodegeneration: Lessons from Alzheimer's Disease and Wolfram Syndrome

Calcium ions (Ca²⁺) operate as important messengers in the cell, indispensable for signaling the underlying numerous cellular processes in all of the cell types in the human body. In neurons, Ca²⁺ signaling is crucial for regulating synaptic transmission and for the processes of learning and memory formation. Hence, the dysregulation of intracellular Ca²⁺ homeostasis results in a broad range of disorders, including cancer and neurodegeneration. A major source for intracellular Ca²⁺ is the endoplasmic reticulum (ER), which has close contacts with other organelles, including mitochondria. In this review, we focus on the emerging role of Ca²⁺ signaling at the ER–mitochondrial interface in two different neurodegenerative diseases, namely Alzheimer’s disease and Wolfram syndrome. Both of these diseases share some common hallmarks in the early stages, including alterations in the ER and mitochondrial Ca²⁺ handling, mitochondrial dysfunction and increased Reactive oxygen species (ROS) production. This indicates that similar mechanisms may underly these two disease pathologies and suggests that both research topics might benefit from complementary research.

The PLEKHA7-PDZD11 complex regulates the localization of the calcium pump PMCA and calcium handling in cultured cells

The plasma membrane calcium ATPase (PMCA) extrudes calcium from the cytosol to the extracellular space to terminate calcium-dependent signaling. Although the distribution of PMCA is crucial for its function, the molecular mechanisms that regulate the localization of PMCA isoforms are not well understood. PLEKHA7 is implicated by genetic studies in hypertension and the regulation of calcium handling. PLEKHA7 recruits the small adapter protein PDZD11 to adherens junctions, and together they control the trafficking and localization of plasma membrane associated proteins, including the Menkes copper ATPase. Since PDZD11 binds to the C-terminal domain of b-isoforms of PMCA, PDZD11 and its interactor PLEKHA7 could control the localization and activity of PMCA. Here, we test this hypothesis using cultured cell model systems. We show using immunofluorescence microscopy and a surface biotinylation assay that KO of either PLEKHA7 or PDZD11 in mouse kidney collecting duct epithelial cells results in increased accumulation of endogenous PMCA at lateral cell–cell contacts and PDZ-dependent ectopic apical localization of exogenous PMCA4x/b isoform. In HeLa cells, coexpression of PDZD11 reduces membrane accumulation of overexpressed PMCA4x/b, and analysis of cytosolic calcium transients shows that PDZD11 counteracts calcium extrusion activity of overexpressed PMCA4x/b, but not PMCA4x/a, which lacks the PDZ-binding motif. Moreover, KO of PDZD11 in either endothelial (bEnd.3) or epithelial (mouse kidney collecting duct) cells increases the rate of calcium extrusion. Collectively, these results suggest that the PLEKHA7–PDZD11 complex modulates calcium homeostasis by regulating the localization of PMCA.

Imaging mitochondrial calcium dynamics in the central nervous system

Mitochondrial calcium handling is a particularly active research area in the neuroscience field, as it plays key roles in the regulation of several functions of the central nervous system, such as synaptic transmission and plasticity, astrocyte calcium signaling, neuronal activity… In the last few decades, a panel of techniques have been developed to measure mitochondrial calcium dynamics, relying mostly on photonic microscopy, and including synthetic sensors, hybrid sensors and genetically encoded calcium sensors. The goal of this review is to endow the reader with a deep knowledge of the historical and latest tools to monitor mitochondrial calcium events in the brain, as well as a comprehensive overview of the current state of the art in brain mitochondrial calcium signaling. We will discuss the main calcium probes used in the field, their mitochondrial targeting strategies, their key properties and major drawbacks. In addition, we will detail the main roles of mitochondrial calcium handling in neuronal tissues through an extended report of the recent studies using mitochondrial targeted calcium sensors in neuronal and astroglial cells, in vitro and in vivo.

Astroglial ER-mitochondria calcium transfer mediates endocannabinoid-dependent synaptic integration

Intracellular calcium signaling underlies the astroglial control of synaptic transmission and plasticity. Mitochondria-endoplasmic reticulum contacts (MERCs) are key determinants of calcium dynamics, but their functional impact on astroglial regulation of brain information processing is unexplored. We found that the activation of astrocyte mitochondrial-associated type-1 cannabinoid (mtCB₁) receptors determines MERC-dependent intracellular calcium signaling and synaptic integration. The stimulation of mtCB₁ receptors promotes calcium transfer from the endoplasmic reticulum to mitochondria through a specific molecular cascade, involving the mitochondrial calcium uniporter (MCU). Physiologically, mtCB₁-dependent mitochondrial calcium uptake determines the dynamics of cytosolic calcium events in astrocytes upon endocannabinoid mobilization. Accordingly, electrophysiological recordings in hippocampal slices showed that conditional genetic exclusion of mtCB₁ receptors or dominant-negative MCU expression in astrocytes blocks lateral synaptic potentiation, through which astrocytes integrate the activity of distant synapses. Altogether, these data reveal an endocannabinoid link between astroglial MERCs and the regulation of brain network functions.

CREB regulates the expression of type 1 inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) play a central role in regulating intracellular Ca2+ signals in response to a variety of internal and external cues. Dysregulation of IP3R signaling is the underlying cause for numerous pathological conditions. It is well established that the activities of IP3Rs are governed by several post-translational modifications, including phosphorylation by protein kinase A (PKA). However, the long-term effects of PKA activation on expression of IP3R subtypes remains largely unexplored. In this report, we investigate the effects of chronic stimulation and tonic activity of PKA on the expression of IP3R subtypes. We demonstrate that expression of the type 1 IP3R (IP3R1) is augmented upon prolonged activation of PKA or upon ectopic overexpression of cyclic AMP-response element-binding protein (CREB) without altering IP3R2 and IP3R3 abundance. By contrast, inhibition of PKA or blocking CREB diminished IP3R1 expression. We also demonstrate that agonist-induced Ca2+-release mediated by IP3R1 is significantly attenuated upon blocking of CREB. Moreover, CREB - by regulating the expression of KRAS-induced actin-interacting protein (KRAP) - ensures correct localization and licensing of IP3R1. Overall, we report a crucial role for CREB in governing both the expression and correct localization of IP3R1. This article has an associated First Person interview with the first author of the paper.

Inositol 1,4,5-trisphosphate receptors in the endoplasmic reticulum: A single-channel point of view

As an intracellular Ca(2+) release channel at the endoplasmic reticulum membrane, the ubiquitous inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) plays a crucial role in the generation, propagation and regulation of intracellular Ca(2+) signals that regulate numerous physiological and pathophysiological processes. This review provides a concise account of the fundamental single-channel properties of the InsP3R channel: its conductance properties and its regulation by InsP3 and Ca(2+), its physiological ligands, studied using nuclear patch clamp electrophysiology.

Calpain-cleaved type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1) has InsP(3)-independent gating and disrupts intracellular Ca(2+) homeostasis

The type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1) is a ubiquitous intracellular Ca(2+) release channel that is vital to intracellular Ca(2+) signaling. InsP(3)R1 is a proteolytic target of calpain, which cleaves the channel to form a 95-kDa carboxyl-terminal fragment that includes the transmembrane domains, which contain the ion pore. However, the functional consequences of calpain proteolysis on channel behavior and Ca(2+) homeostasis are unknown. In the present study we have identified a unique calpain cleavage site in InsP(3)R1 and utilized a recombinant truncated form of the channel (capn-InsP(3)R1) corresponding to the stable, carboxyl-terminal fragment to examine the functional consequences of channel proteolysis. Single-channel recordings of capn-InsP(3)R1 revealed InsP(3)-independent gating and high open probability (P(o)) under optimal cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) conditions. However, some [Ca(2+)](i) regulation of the cleaved channel remained, with a lower P(o) in suboptimal and inhibitory [Ca(2+)](i). Expression of capn-InsP(3)R1 in N2a cells reduced the Ca(2+) content of ionomycin-releasable intracellular stores and decreased endoplasmic reticulum Ca(2+) loading compared with control cells expressing full-length InsP(3)R1. Using a cleavage-specific antibody, we identified calpain-cleaved InsP(3)R1 in selectively vulnerable cerebellar Purkinje neurons after in vivo cardiac arrest. These findings indicate that calpain proteolysis of InsP(3)R1 generates a dysregulated channel that disrupts cellular Ca(2+) homeostasis. Furthermore, our results demonstrate that calpain cleaves InsP(3)R1 in a clinically relevant injury model, suggesting that Ca(2+) leak through the proteolyzed channel may act as a feed-forward mechanism to enhance cell death.

Unitary Ca(2+) current through recombinant type 3 InsP(3) receptor channels under physiological ionic conditions

The ubiquitous inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) channel, localized primarily in the endoplasmic reticulum (ER) membrane, releases Ca(2+) into the cytoplasm upon binding InsP(3), generating and modulating intracellular Ca(2+) signals that regulate numerous physiological processes. Together with the number of channels activated and the open probability of the active channels, the size of the unitary Ca(2+) current (i(Ca)) passing through an open InsP(3)R channel determines the amount of Ca(2+) released from the ER store, and thus the amplitude and the spatial and temporal nature of Ca(2+) signals generated in response to extracellular stimuli. Despite its significance, i(Ca) for InsP(3)R channels in physiological ionic conditions has not been directly measured. Here, we report the first measurement of i(Ca) through an InsP(3)R channel in its native membrane environment under physiological ionic conditions. Nuclear patch clamp electrophysiology with rapid perfusion solution exchanges was used to study the conductance properties of recombinant homotetrameric rat type 3 InsP(3)R channels. Within physiological ranges of free Ca(2+) concentrations in the ER lumen ([Ca(2+)](ER)), free cytoplasmic [Ca(2+)] ([Ca(2+)](i)), and symmetric free [Mg(2+)] ([Mg(2+)](f)), the i(Ca)-[Ca(2+)](ER) relation was linear, with no detectable dependence on [Mg(2+)](f). i(Ca) was 0.15 +/- 0.01 pA for a filled ER store with 500 microM [Ca(2+)](ER). The i(Ca)-[Ca(2+)](ER) relation suggests that Ca(2+) released by an InsP(3)R channel raises [Ca(2+)](i) near the open channel to approximately 13-70 microM, depending on [Ca(2+)](ER). These measurements have implications for the activities of nearby InsP(3)-liganded InsP(3)R channels, and they confirm that Ca(2+) released by an open InsP(3)R channel is sufficient to activate neighboring channels at appropriate distances away, promoting Ca(2+)-induced Ca(2+) release.